1. Technical Field
The present invention relates generally to semiconductor devices and in particular to a semiconductor capacitor. Still more particularly, the present invention relates to a linear capacitor and a process for fabricating the same.
2. Description of the Related Art
Linear complementary metal oxide semiconductor (CMOS) products can benefit greatly from the availability of linear capacitors in which both terminals are isolated from the substrate and the voltage coefficient of capacitance in both directions are very small. Linear capacitors are useful for many applications, including filters, charge-redistribution networks, and compensation in standard two-stage amplifiers. The industry "workhorse" for linear capacitors in recent years has been the "poly-poly" or "polysilicon to polysilicon" capacitor. Integration of such capacitors has become increasingly difficult because the polysilicon doping is now often dictated by considerations in fabricating other devices.
In FIGS. 1A-1C, a known process for fabricating a linear capacitor is shown. In FIG. 1A, a substrate 100 with field oxide region 102 is formed thereon as shown. Substrate 100 is a semiconductor substrate, such as, silicon. Gate polysilicon layer 104 has been formed on exposed substrate 100 and field oxide region 102. An N+ implant or diffusion is performed on this layer.
In FIG. 1B, an oxide layer 106 is formed that will create a poly-poly oxide layer for the linear capacitor. Thereafter, a second polysilicon layer is deposited, doped, and etched to form polysilicon top-plate 108 for the linear capacitor. The doping that forms polysilicon top-plate 108 is performed as part of a N+ source/drain implant. Doping of polysilicon top-plate 108 may be realized by defusion, implantation, or doping as this layer is deposited.
Thereafter, in FIG. 1C, exposed portions of oxide layer 106 are stripped and polysilicon layer 104 is patterned and etched to form gate structure 110 and polysilicon bottom-plate 112. Polysilicon bottom plate 112 forms the bottom portion of the linear capacitor.
In FIGS. 2A-2C, diagrams illustrating a known process for fabrication of a transistor and a linear capacitor are depicted. In FIG. 2A, substrate 200 is shown with field oxide region 202. A polysilicon layer has been deposited, patterned, and etched to form polysilicon bottom-plate 204.
Next, in FIG. 2B, gate oxidation is performed to form poly-poly oxide layer 206 for the linear capacitor. A second polysilicon layer is deposited, patterned, and then implanted with a N+ source/drain implant to form source/drain regions 208 and 210. Then the second polysilicon layer is etched to form polysilicon top-plate 212 and gate structure 214 as illustrated in FIG. 2B. The etching process also forms spacers 215 and 216, which are artifacts of the etching process.
If the poly-poly oxide is not formed at the same time as the gate oxidation, another photocut is required to strip active regions for gate oxidation. Additionally, the doping in polysilicon top-plate 212 is doped in the same step as that for gate structure 214. The amount of doping of the polysilicon layer forming polysilicon top gate is defined to optimize transistor and resistor performance, such as gate structure 214. The resistance with the doping is typically 70 ohms to 150 ohms, which is not enough resistance for the capacitor. Low enough doping levels result in nonlinearities in capacitance. In other words, the capacitance changes when a direct current voltage is applied to the capacitor. Therefore, it would be advantageous to have an improved linear capacitor.